The invention generally relates to electronic control systems used in controlling liquid heating apparatus for raising the temperature of connected bodies of water. More particularly, the present invention relates to a control system which controls a heat pump which is coupled in heat exchange relationship to a domestic water heater, or other body of water to be heated such as a spa. Such a heat pump may be self-contained with a hot water retention tank provided therein and may be either an air-to-water type unit, a water-to-water type unit or a direct exchange (DX)-to-water type unit.
It is known to replace or augment conventional electric resistance water heaters with heat pump water heaters as a more efficient means of producing domestic hot water. One prior art method of controlling such heat pump water heaters has been to use two-position bimetallic type thermostats which are generally provided in domestic water heaters as the primary operating control. An example of such a prior art heat pump water heater control circuit may be found at U.S. Pat. No. 5,255,338 (Robinson, Jr. et al). One advantage of this type of arrangement is that the hot water thermostat is located directly in the hot water tank.
A disadvantage associated with the above described method is that two-position bimetallic type thermostats are not as versatile as full range type sensors, such as thermistors, and are not as effective when used with microprocessor type controls. Another disadvantage is that tying into the water heater thermostat and control wiring often results in voiding UL or other industry certifications. Other prior art systems have placed sensors directly in the hot water tank, which results in the disadvantage of added retrofit labor and material costs and, again, the possibility of voiding certifications.
While the comparison of energy costs between heat pump type water heaters versus electric resistance type water heaters favors the use of the heat pump, one detraction from the use of heat pump type water heaters is the issue of quick heat recovery. In keeping the costs of heat pump type water heaters comparable with the costs of electric resistance type water heaters, manufacturers have tried to minimize the size of the compressor used in heat pump type water heaters. An unfortunate result of this is the reduced heating capacity of the heat pump water heater unit. While a typical electric resistance type water heater will deliver 16,000 BTU's per hour of heating capacity, a typical heat pump type water heater has a much reduced heating capacity of 7,000 BTU's per hour. Accordingly, when a large demand for hot water consumes the hot water stored in the hot water retention tank, the electric resistance type water heater is able to more rapidly heat the replacement cold water than a typical heat pump type water heater.
For consumer satisfaction, a quick heat recovery rate is essential. For this reason heat pump type water heaters are most often used in conjunction with conventional electrical resistance type water heaters. The electric resistance heating elements are generally used to compliment the heat pump water heating capacity during periods of large demand. Another problem typically associated with heat pump water heaters is that of liming, which effectively reduces the capacity of the unit and may eventually lead to compressor damage. A prior art method of preventing compressor damage due to liming was to include a high pressure switch to terminate compressor operation upon excessive pressure being exhibited in the heat pump system. A draw back associated with this is that the compressor is shut down, often prematurely, with no advance warning and a service call is required to place the unit in an operating condition.
Another condition associated with heat pump operation is that of high evaporator temperature, which corresponds to high suction pressure. Generally, the suction side high pressure limit for a heat pump water heater type compressor is 90 PSIG, this corresponds to an evaporator refrigerant discharge temperature of 62.degree.. In the case of an earth ground loop system operating under summer conditions, the loop temperature will often be in the range of 80.degree.-100.degree. F. or above. This results in elevating the evaporator refrigerant discharge temperature above 62.degree. F. and tripping the high suction pressure limit switch. Prior art heat pump water heaters coupled to a ground loop system simply lock out compressor operation based upon a high pressure limit switch located on the suction side of the compressor. In the case of air-to-water type heat pump units, freeze protection on the evaporator coil is of prime importance. Prior art heat pump water heaters utilize a two-position bimetallic type thermostat which locks out compressor operation upon experiencing a freeze condition at the intake of the evaporator coil. To place the heat pump unit in a condition for operation, a service call was necessary or at least a resetting of the freeze-stat by maintenance personnel.